1. Field of the Invention
The present invention is concerned with processing erbium to obtain a lower average thermal neutron capture cross section by a partial or complete separation of its isotopes thus improving its suitability as an internal material of construction, for instance, as a fuel rod cladding, for a nuclear reactor.
2. Background Information
The role of erbium (Er) in nuclear fuel assembly design is as a burnable poison with high neutron capture cross-section. Its properties are comparable or superior to boron. Its chemical properties are similar to those of gadolinium (Gd) except that Er(III) is a slightly more acidic ion than Gd(III) in solution; therefore, its separation chemistry should be somewhat more powerful than that of Gd in a similar series of eluents.
The key issue in designing an enrichment system for the Er isotopic enrichment chemistry is that the key fractions of the high cross-section isotopes are embedded in undesirable isotopes. This results in the requirement of multiple extraction trains when using separations technologies such as solvent extraction or fixed bed ion exchange columns which generically produce only "heads" and "tails" fractions but nothing in the "middle". Hence, multiple trains are required to essentially dissect the fractions until the desired head or tail emerges from the column as the sole constituent of the product fraction. When combining this enrichment issue with the fact that Er separation from the ores requires both complex chemistry and hardware, one arrives at the summary of issues that must be addressed by a process technology that recovers Er from the raw ore and refines it to an isotopically pure product for inclusion in nuclear fuel assembly design:
a. Separation of erbium from the other rare-earth elements and impurities in the ore leachate
1. requires a large number of multi-stage extraction circuits due to relatively low specificity of distribution coefficients for most extraction solvents; PA1 2. requires a large number of large, awkward, expensive stages per extraction circuit due to the low magnitude of the distribution coefficients; PA1 3. generates hazardous and mixed wastes as a result of the organic based solvents; and PA1 4. requires significant peripheral purification operations to refine the erbium once it is separated from the other rare-earths and to prepare it for isotopic enrichment. PA1 1. inherent batchwise operation; PA1 2. awkward valving required for product recovery when columns are operated in the chromatographic mode; PA1 3. complex process control problems associated with process operation and wave front separation; and PA1 4. excessive product dilution associated with column operation, and others.
b. Lack of a powerful, compact, cost-effective enrichment process which, again, generates no hazardous waste as a byproduct. Solvent extraction is ineffective due to the inherently low separation alpha's (driven by the fact that trivalent ions do not form readily separable solution complexes), and to its large, capital-intensive, separation stages that increase both operating and maintenance costs, generate hazardous process wastes (and possibly hazardous/radioactive mixtures depending on the composition of the original feed) and decrease process availability.
c. There is a lack of a mechanically-feasible continuous ion exchange contactor design to overcome the historical problems associated with ion exchange. These historical problems are: